Printing apparatus and methods have been provided for the ink printing of images. Most commercial printing today is done by relief printing, offset lithography or gravure printing. Briefly, the printing surface is raised above the level of the nonprinting surface in relief printing, while the printing surface is below the nonprinting surface in gravure printing. In offset lithography, or planography, the printing and nonprinting surfaces are on the same level.
In one form of relief printing, a flexible printing plate having a photosensitive plastic over a metal base is mounted to a rotatable cylinder. A negative of the image to be printed is properly aligned and exposed to the plastic causing the image area to harden. The nonimage areas remain soft and can be removed, leaving the image areas in relief. The plate is hardened before being attached to the cylinder. Ink is applied to the plate and impressed by the image areas against printing stock passing below the rotating cylinder.
In offset lithography, the most commonly used printing process, the image to be printed is reproduced on a printing plate by a process known as photolithography. In this process, a negative of the image is held up against the surface of a printing plate which has a light sensitive coating thereon. The negative is exposed to light causing the light sensitive coating to harden in the image areas produced thereon. After removing the coating from the nonimage areas and treating the plate with lacquer, which adheres only to the hardened image areas, the nonimage areas are gummed to increase their ability to hold water.
Rotary presses are typically used in lithography to print the image on the printing stock. The printing plate is clamped to a cylinder known as the plate cylinder. As the plate cylinder rotates, the printing plate first engages a water roller which wets the gummed areas so that they will repel ink. The printing plate next engages an ink roller which applies ink to the image areas. The inked image is transferred to a second cylinder known as the blanket cylinder, which in turn transfers the image to the printing stock.
Color images can be reproduced in relief printing, lithography or gravure by a procedure known as process color printing. In general, printing presses use the method of "subtractive" mixing to print full color images. In this method, the picture is not the source of light but merely reflects the desired light. The color image to be printed is created by combining tiny dots of yellow, magenta and cyan on the printing stock. (Black is often used as well to add sharpness to the print.) Each dot creates a certain color of light by absorbing certain colors in the white light falling on it and reflecting other colors. For example, a yellow surface absorbs the blue light in the white light falling on it and reflects the colors red and green to create the color yellow. The light reflected by each dot, together with the light reflected by the adjoining dots, creates the color at that location on the printed image when the dots are viewed from a distance as to not be individually discernable. Accordingly, the presence and size of a dot, together with the presence and size of the other single-color dots which are nearby, determines the color at that location on the image. An eye viewing the printing stock with the tiny colored dots thereon mixes the colors of the dots into all the colors and shades of the original copy.
The single-color dot configured images, known as halftones, of the image to be printed are created by separately photographing the image through a series of filters, each of which blocks out all but the desired color. Each of these photographs is taken through a distinctly configured halftone screen to produce a halftone negative with a dot configuration different from the other halftone negatives. The halftone negatives are used to make separate printing plates for each of these colors. Each printing plate is used with a separate set of cylinders and has its own supply of colored ink. The printing stock passes sequentially through the color printing cylinders, which are arranged so that each halftone image is properly registered with respect to the others.
As discussed above, current color ink printing technology requires a separate printing pass for each of the primary colors. Each pass adds a layer of redundancy to the operation in the form of additional presses and printing plates and is reflected in the cost of color printing. The requirement that each halftone image be properly registered with the others adds a layer of complexity to the standard printing process. Failure to properly register the halftone images results in prints of inferior quality. On the other hand, any new and improved printing technology which requires the replacement of existing presses may be difficult to sell in the already capital intensive and highly competitive print industry.
Apparatus and methods have also been provided for creating color images without the use of ink. Instead, the optical properties of the surface on which the image is created are used to recreate color images through the process of "additive" mixing. In this process, the image is the source of light and typically reflects the primary colors blue, red and green. A pattern of diffraction gratings are created on the printing surface, similar to the pattern of yellow, magenta and cyan dots used to create colors by subtractive mixing, to cause white light falling on the surface to reflect the primary colors. Each location on the image reflects the desired primary color(s) which are combined with primary colors reflected by the adjoining diffraction gratings to produce the desired color at that location on the image. White light is reflected at locations where the primary colors are reflected in approximately equal density. The image does not absorb or subtract colors to produce the desired color.
Unfortunately, existing apparatus and methods for creating color images through additive mixing are expensive. Many require that an expensive embossing plate be custom made with lasers for each image to be printed. These apparatus and methods utilize the process of hot foil stamping to create the color images, and do not utilize standard high volume rotary printing processes.